In electrophotography (sometimes more generally referred to as electrostatography), an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image) can be formed on a surface of an electrophotographic element and is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development. The toner image is eventually transferred to a receiver, to which it is fused, typically by heat and pressure.
Toners typically contain a binder and other additives, such as colorants. Binders are generally polymeric and are selected so as to provide a balance between various conflicting constraints. One of the most common additives that is used in toner compositions is a release additive such as low molecular weight polyolefin waxes or fatty acids or fatty amides or salts thereof. These "release additives" are present to impart better release of the toner melt from the fuser roller surface during the fusing process. This is achieved with the combination of a low surface energy roller, e.g., a polytetrafluoroethylene coated roller, and a toner that contains preferably 1-2% by weight of the release additives in the toner composition. Examples of patents that disclose these release additives in toner compositions include: GB 1,570,239; JP A 63-55563; and JP A 62-28770.
Toner compositions having release additives are made by melt compounding and pulverization. This process produces a wide particle size distribution and poor particle shapes leading to the less than desired image quality.
While the images that are made with the toner compositions having release additives are acceptable in many respects, they have less than desired abrasion resistance and less than desired image quality because of the irregular shape of the toner particles and because of the wide particle size distribution. Further, where these toner compositions are used to make glossy images, these images are susceptible to finger prints and damage when placed against a plasticized vinyl surface.
In U.S. Pat. No. 4,643,960, there is described a method wherein the irregular shaped powder is further processed by aspirating it into a moving gas stream. The aerosol produced is directed through a stream of hot gas and into a cooling chamber. This produces polydisperse spherical particles. These spherical particles of binder are then dry blended with pigment to produce a conductive toner composition. A small amount of fatty acid amide, e.g., 0.05 to 5% by weight, can be used on the surface of the particles to facilitate the dry blending process. No charge control agents are used. U.S. Pat. No. 4,745,418 is similar.
U.S. Pat. Nos. 4,833,060; 4,835,084; 4,965,131; 5,049,469 and 5,133,992 all teach the methods of preparing making toners using colloidally stabilized suspension polymerization or evaporation processes. By using the techniques described in these patents, it is possible to formulate narrow particle size distribution toners by utilizing either polymerization or evaporation limited coalescence methods. In the case of polymerization limited coalescence technique, a mixture of monomers, wherein the desired pigment (where present) dispersion has been incorporated along with appropriate charge agents, polymerization initiator, chain transfer agents is colloidally stabilized in an aqueous media. The stabilized particles are then polymerized under appropriate conditions and the resulting toner particles isolated by various procedures as described in these patents.
In the evaporative limited coalescence technique, an organic solvent solution of preformed binder polymer, wherein the desired pigment (where present) dispersion and charge agents have been incorporated, is colloidally stabilized in an aqueous media. The desired toner particles are formed once the organic solvent is allowed to evaporate and the subsequent particles isolated. These toners can now be used directly in an electrophotographic process without any further processing such as melt compounding.
These techniques are very useful for preparing toner with narrow particle size distributions. They are particularly useful for preparing small toner particles that are less than 7 micrometers volume average diameter in size and composition which have no particles less than 2 micrometers. However, these techniques are not useful if a release additive such as low molecular weight polyolefins, etc., are required to be added to the toner formulation. In conventional melt compounding and pulverizing methods where release additives can be added, the process of melt kneading the toner binder, pigments and charge agents, etc., is carried out at sufficiently high temperatures to permit melting of any low surface energy release additive. Since the limited coalescence processes are carried out at ambient or low temperatures, the incorporation of release additives by melting is not possible. Thus, prior to the present invention, it has not been possible to obtain monodisperse spherical toner particles with any significant amount of release additives. In particular, it is not feasible to uniformally incorporate any significant amount of an aliphatic amide or aliphatic acid by incorporating it into the polymerizable monomer before suspension polymerization, or by other methods. (See U.S. Pat. No. 5,133,992, col 10 lines 29-36.)
Other toner compositions are known which contain high quantities of the low molecular weight polyolefin waxes or fatty acids or fatty amides or salts thereof. In this case, these components are present not to provide improved release but rather to provide a wax binder for "pressure fixing". In this process, pressure is used to fix the latent image onto the receiver sheet. Heat is not necessary and relatively large amounts of the waxy substance are needed to provide this effect. As a result of the high level of this waxy binder, the toner image itself has an undesirable waxy character. Processes of this type are disclosed in U.S. Pat. Nos. 4,100,087; and 4,745,418.
There is a continuing need for toner compositions that have desirable image characteristics as well as improved release properties and abrasion resistance.